Human immunity to cancer can be assayed by tube leukocyte adherence inhibition (LAI). The assay is used to monitor the isolation of the sensitizing antigen, an organ-specific neoantigen. We have purified a small quantity of a single polypeptide chain carrying the OSN epitope of colorectal cancer from urine and solid tumors of patients with metastatic cancer. Most patients with small invasive cancers have detectable antitumor immunity. This means that immunologic assays of antitumor immunity can detect patients with curable cancer. Unfortunately, cellular assays of antitumor immunity, like LAI, are not exploitable by routine diagnostic laboratories. We plan to purify large quantities of the colorectal OSN in order to develop serologic assays. The OSN from the urine of patients with metastatic colorectal cancer will be purified by physicochemical and affinity chromatography procedures which we established when purifying a small quantity of the OSN. Fifty liters of urine from patients with metastatic colorectal cancer yield about 2 mg of pure OSN. The purified OSN will be characterized by peptide mapping and NH2-terminal amino acid sequence analysis; its structure is to be compared with other tumor antigens isolated by similar methods and to tumor antigens isolated directly from the cancer cell membrane and with other proteins to which it might be related such as HLA antigens. The OSN will be used to raise polyclonal and monoclonal antibodies for use in the development of enzyme-linked immunosorbent assays for human antitumor antibody and circulating tumor antigen. It is now possible to reliably predict the presence or absence of cancer by the tube LAI assay. If the components of this assay can be converted into a simple serologic assay, we believe that the resultant assays would have a significant impact on the detection and subsequent cure of cancer.